Rotary timing valve

ABSTRACT

A hydrostatic transmission having axial piston variable displacement pump and motor units having a timing valve cooperating with each main port to control the piston stroke pressure change to equalize the pressure in each cylinder and the pressure in each main port at the moment of interconnection. In the pump, as the cylinder port rotates from the high pressure port past top dead center to the low pressure port, one timing valve between top dead center and the leading edge of the low pressure port acts as a predecompression valve and as the cylinder port moves from the low pressure port past bottom dead center to the high pressure port the other timing valve between bottom dead center and the leading edge of the high pressure port acts as a precompression valve. Reversing the pump tilt box from a forward position to a reverse position with the same direction of rotation reverses the high and low pressure ports and the decompression and compression functions of the timing valves. In the motor with the cylinder rotating in the opposite direction relative to the valve plate and the tilt box in a forward position, the top dead center timing valve is a compression valve at the trailing edge of the low pressure port and the bottom dead center timing valve is a decompression valve at the trailing edge of the high pressure port. The timing valves in one construction are rotatable balanced valves and in another construction reciprocating valves balanced by a corresponding control force. The compression and decompression valves respectively control the amount of precompression and predecompression by piston stroke to equalize the cylinder pressure and the main port pressure as the connection is made in a pump and a motor for silent efficient operation. The compression valves are automatically controlled to increase precompression as a function of increasing system pressure and decreasing displacement and speed. The decompression valves are automatically controlled to increase predecompression as a function of increasing system pressure and speed and of decreasing displacement.

United States Patent [19] Week et al.

[4 1 Sept. 16, 1975 ROTARY TIMING VALVE [75] Inventors: Nils P. Week, Allen Park; Carl E.

Shellman; Erkki A. Koivunen, both of Livonia, all of Mich.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

[22] Filed: Jan. 28, 1974 21 Appl. No.: 437,475

Primary ExaminerWilliam L. Freeh Assistant ExaminerG. P. LaPointe Attorney, Agent, or FirmA. M. Heiter [5 7 ABSTRACT A hydrostatic transmission having axial piston variable displacement pump and motor units having a timing valve cooperating with each main port to control the piston stroke pressure change to equalize the pressure in each cylinder and the pressure in each main port at the moment of interconnection. 1n the pump, as the cylinder port rotates from the high pressure port past top dead center to the low pressure port, one timing valve between top dead center and the leading edge of the low pressure port acts as a predecompression valve and as the cylinder port moves from the low pressure port past bottom dead center to the high pressure port the other timing valve between bottom dead center and the leading edge of the high pressure port acts as a precompression valve. Reversing the pump tilt box from a forward position to a reverse position with the same direction of rotation reverses the high and low pressure ports and the decompression and compression functions of the timing valves. 1n the motor with the cylinder rotating in the opposite direction relative to the valve plate and the tilt box in a forward position, the top dead center timing valve is a compression valve at the trailing edge of the low pressure port and the bottom dead center timing valve is a decompression valve at the trailing edge of the high pressure port. The timing valves in one construction are rotatable balanced valves and in another construction reciprocating valves balanced by a corresponding control force. The compression and decompression valves respectively control the amount of precompression and predecompression by piston stroke to equalize the cylinder pressure and the main port pressure as the connection is made in a pump and a motor for silent efficient operation. The compression valves are automatically controlled to increase precompression as a function of increasing system pressure and decreasing displacement and speed. The decompression valves are automatically controlled to increase predecompression as a function of increasing system pressure and speed and of decreasing displacement.

39 Claims, 10 Drawing Figures PATENTED SEP 1 EH75 SHUIT 3 BF 6 M r wk PATENTEU SEP 1 s 1975 SHEET 5 or g ROTARY TIMING vALvE RELATED APPLICATIONS SUMMARY OF THE INVENTION 1n hydrostatic transmissions having pump and motor units timing valves are provided between each dead center position and an adjacent main port to equalize the pressure in each cylinder and main port at the time they are interconnected. The pump and motor units are preferably structurally identical axial piston variable displacement units with the pistons reciprocated in cylinders in a rotary cylinder block by a tilt box or swash plate and the cylinders having cylinder ports in the end surface of the cylinder block in face sealing bearing contact with the stationary valve plate so the cylinder ports are alternately connected to the main ports in the valve plate. The pump and motor units also preferably have identical timing valves located in the valve plate between each dead center position of the cylinder port and one end edge of an adjacent main port to control the amount of dead center movement, which is cylinder movement while the cylinder port is disconnected from both main ports, and thus the amount of piston stroke effected pressure change in each cylinder for equalizing cylinder and its port pressure with the main port pressure at the time of interconnection.

In the pump as the cylinder port rotates from the high pressure main port past top dead center to the low pressure main port, one timing valve between top dead center and the leading edge of the low pressure main port is movable between a minimum and a maximum position to control the amount of top dead center movement, movement past top dead center, for stroke induced pressure reduction or functions as a predecompression valve. As a pump cylinder port rotates from the low pressure main port past bottom dead center to the high pressure main port, the other timing valve between bottom dead center and the leading edge of the high pressuremain port is movable between a minimum and a maximum position to control the amount of bottom dead center movement, movement past bottom dead center, for stroke induced pressure increase or functions as a precompression valve. When the pump tilt box is reversed, moved between forward and reverse positions, the top and bottom dead centers, the low and high pressure in the main ports and the precompression and predecompression functions of the timing valve are reversed.

In the motor with cylinder rotating in the opposite direction relative to the valve plate, as compared to pump rotation, and with the tilt box in a drive position, like the forward pump position, the top dead center timing valve is a precompression valve between the trailing edge of the low pressure port and top dead center position of the cylinder port controlling the amount of stroke effected pressure increase to equalize the cylinder and its port pressure and the high main port pressure at the time they are interconnected or immediately after the cylinder port moves past top dead center. The bottom dead center timing valve is a predecompression valve between the trailing edge of the high pressure port and the bottom dead center position of the cylinder port controlling the amountof stroke effected pressure decrease to equalize the cylinder and its port pressure and the high main port pressure at the time they are interconnected or immediately after the cylinder port moves past bottom dead center.

The timing valves are rotatably mounted in the valve plate and have a bearing surface aligned with the valve plate bearing surface and similarly contacting the bearing surface on the cylinder block. Each rotary timing valve has a recess operative in minimum position to extend from the adjacent main port end, the leading port in a pump and the trailing port in a motor, to or almost to the dead center cylinder port position so there is minimum dead center or pressure change movement. Timing valve rotation to maximum position rotates the recess toward and to the adjacent main port and so dead center or pressure change movement is increased to a maximum. The recess in the cylindrical valve is pressure balanced so system pressure does not exert a rotary force. A reciprocating timing valve slides in a bore connected to the end of the port spaced from the dead center position. A series of openings connect the valve plate face to the bore in the area traversed by the cylinder port. The reciprocating timing valve in minimum position opens all the openings and on movement to maximum position sequentially closes the openings to increase the dead center movement and pressure change.

Automatic control of the timing valves for equalizing cylinder and main port pressures is basically a function high forward or reverse system pressure acting to increase compression and decompression and opposed by a spring bias. A displacement signal pressure proportional to the amount of displacement for either forward or reverse acts to reduce compression and decompression. The pressure in the main ports adjacent each timing valve is connected to the control for the adjacent timing valve to act to increase precompression and predecompression. In this way precompression is increased by high main port pressure and predecompression is increased by high and low main port pressure in both forward and reverse operation. Each timing valve when providing precompression provides more precompression than the predecompression provided by the other timing valve, since precompression is reduced by leakage and predecompression is increased by leakage. The governor pressure signal decreases precompression proportional to unit speed and increases predecompression proportional to unit speed to accommodate for the reduced time leakage occurs at higher speeds.

in the pump a control valve controlled by the forward displacement pressure and the reverse displacement pressure selectively supplies governor signal pressure to provide a forward governor signal and a reverse governor signal to reverse the governor control action on the tirriing valves on a forward reverse shift when the predecompression and precompression functions of these valves are reversed.

These and other features of the invention will be more apparent from the description and the drawings in which: 7

FIG. I is a sectional view showing a pump or motor hydrostatic unit.

FIG. 2 is a detail view of the biasing pistons.

FIG. 3 is a detail view of a governor valve.

FIG. 4 is a view of the valve plate on section line 44 of FIG. 1 and the connecting ports in the housing.

FIG. 5 is an end of FFG. l on line 55.

FIG. 6 is a partial schematic diagram of a pump motor hydrostatic transmission showing the power system and displacement controls.

FIGS. 7a and 7b when placed together as shown in FIG. 8 is a partial schematic diagram of the timing valves and their controls and with FIG. 6 schematically the complete hydrostatic transmission and controls.

FIG. 8 shows the assembly of FIGS. 7a and 7b for the composite view.

FIG. 9 is a partial detail view of a modified timing valve in the valve plate.

The hydrostatic converter, a hydrostatic pump or motor unit, shown in FIG. 1, may be used as the pump and motor in the system or transmission shown diagrammatically in FIGS. 6, 7a and 7b. Referring to FIG. 1, the hydrostatic converter has a barrel shaped housing 10, having an integral valve supporting end wall 11 at the valve end and a shaft end support wall 12 at the shaft end suitably secured by bolts and sealed to close the open end of the barrel housing 10. The closure wall 12 has a central opening 14 to receive the adapter, or bushing 15, which is secured by suitable fasteners 16 to the wall 12. The shaft end wall 12 with adapter supports the shaft seal 17, rotary bearing I8, and thrust bearing 19 which rotatably and axially seal, support and locate shaft 21 in the shaft end wall 12. The shaft 21 also extends at its other end into a bore 22 in a cylindrical support member 23 secured and sealed, as described below, in opening 24 in the valve end wall 11. The rear end of shaft 21 is supported by a bearing 26 located in a recess 27 in the bore 22.

On the inside face of the shaft end wall 12, there are formed, or secured, a pair of semi-cylindrical bearing members 31 and 32 having respectively the plane semicylindrical bearing surfaces 33 and 34 which extend vertically and are located on opposite sides of, and equidistant from, the axis 36 of the machine and shaft 21 and are parallel to each other. These plane semicylindrical bearing surfaces 33 on the near side and 34 on the far side of the shaft in FIG. 1 cooperate respectively with bearing surfaces 37 and 38 on the tilt box 39 and may be hydrostatic bearings as described in the above US. Pat. No. 3,779,137 The tilt box 39 is supported by these bearings for rotation about a horizontal axis 41 perpendicular to axis 36. The annular wear or swash plate 43 has an annular bearing surface 44 in engagement with the bearing 42 including an annular and a rim surface on the tilt box to rotatably support and locate the wear plate on the tilt box.

The other annular bearing surface 47 on the wear plate slidably supports the slippers 48 which have a foot portion 49 with a hydrostatic bearing recess 51 within a circular bearing surface engaging the annular bearing surface 47 and a ball portion 52 fitting, and retained in, a socket 53 of the piston 54. The piston 54 has sealing grooves 56 and fits in a cylinder 57 in the rotor, or rotary cylinder block, 58. The block 58 contains a plurality of cylinders with a piston in each cylinder. The illustrative embodiment has nine cylinders spaced about the n'richine axis. The block has a central opening 61 having a splined portion 62 of smaller diameter adjacent the end of the block next to the wear plate. A spherical bearing member 63 is slidably mounted on the shaft and biased by a plurality of coil springs 64 toward the tilt box. The spherical bearing surface has oil grooves and supports the spherical bearing portion 66 of the retainer plate 67. The retainer plate 67 has a recess 68 for each slipper fitting around the central portion of the slipper and engaging an annular surface 69 on the slipper foot to retain the slipper in contact with the wear plate by the bias of springs 64 when fluid pressure in the cylinder does not.

The tilt box 39 is rotated on the bearing 33, 34 about the axis 41 by hydraulic actuators 71 and 72 which respectively actuate the top actuator rod 73 fitting into socket 74 on the tilt box and actuator rod 76 fitting into socket 77 on the lower portion of the tilt box. The hydraulic actuators 71, 72 are identical and each has a piston 78 reciprocatable in a bore 79 and a socket 80 fitting its rod. The bore 79 has an end closure cap 82 threadably secured and sealed to the housing and having a continuation of bore 79 and a threaded port 83 to receive a fitting of a hydraulic line. A spring 84 in each bore abuts the closure cap to resiliently bias the piston to engage the actuator rod in its sockets and to provide a bias positioning the tilt box in the neutral or zero angle position. The controls shown in FIG. 6 sup ply actuator pressure via ports 83 to the bores 79 of the actuators 71 and 72 to position the tilt box to a desired angle. When the tilt box is in a neutral position perpendicular to the unit axis 36 there is no pump or motor operation. When the tilt box is moved clockwise or counterclockwise from neutral, the unit operating as a pump and driven in one direction, provides a pumping action in opposite directions and as a motor receiving fluid supply in one direction provides opposite directions of rotation.

The valve plate 86, FIGS. 1 and 4, is located between the valve end wall 11 of the housing and the rotary cylinder block 58 and has a bearing and sealing surface 87 engaging the bearing and sealing surface 88 on the wall 1 1 and then on the opposite side, a bearing and sealing surface 89 engaging the bearing and sealing surface 91 on the end of the cylinder block. These bearing and sealing surfaces are annular. A port 92 connects each cylinder 57 through the end of the cylinder block and its bearing surface 91. A pin 93 in the end wall 11 fits in a radial slot in the valve plate 86 to prevent rotation of the valve plate 86 and permit its seating on the sealing and bearing surfaces between end wall 1 l and cylinder block 58. The valve plate perimeter and shoulder 94 have bearing engagement to radially locate the valve plate in the housing. A radially inner annular groove 95 and a radially outer annular groove 96 on each opposite face of the valve plate define an annular sealing and bearing surface area or pad and inner and outer pad portions 90' on each side of the valve plate. The inner grooves are vented toward the center and the outer grooves 96 have radial portions for venting to the outer perimeter of the valve plate. The sealing bearing surface 87 nonrotatably engages surface 88 on wall 1 1 and functions as a hydrostatic sealing bearing and may float to insure good axial alignment. The sealing bearing surface 89 since it engages the rotating surface 91 on the cylinder block has pad portions 90 functioning as a hydrokinetic bearing. The ports 92 in the cylinder block 58, the ports 101 and 107 in the valve plate 86 and the main pressure ports 102 and 108 in end wall 11 are within these sealing bearing surfaces to prevent exhaust or limit leakage to a low value to the interior of housing 10.

When the shaft 21 is driven in a clockwise direction, as viewed from the front of the unit, as indicated by the arrow A, FIGS. 1 and 4, and the tilt box is tilted as shown in FIG. 1, the cylinder block 58 rotates in this direction relative to the stationary valve plate 86 and the valves are functional as described for a pump as follows. The lozenge shaped port 101 in the valve plate is the low pressure inlet port and is faired by port 102 in the housing end wall into an inlet or return passage 208. The initially opening end or leading edge 103 of port 102 has a tapered initial small volume slot portion 104 cooperating with the top dead center rotary timing valve 106 located between the slot portion and the cylinder port in top dead center 92a functioning as a predecompession valve. At the other side of the valve plate 86 there is the lozenge shaped high pressure valve plate port 107 which is faired by housing port 108 into the high pressure supply line 207. At the initially opening end or leading edge 109 of port 107,there is also a shallow step or slot portion 111 cooperating with the timing valve '114 located between the slot and the cylinder port at bottom dead center 92b and functioning as a precompression timing valve.

The upper timing valve 106 has a recess sealed rotatable fit in bore 115 in the valve plate and is connected by splines 116 to the shaft 117 rotatably mounted in bore 118 in end wall 11, and sealed by seal 119. The shaft 117 extends through the end wall 11 and is secured and fixed, as by welding, to a radially extending portion of lever 121 and secured in any one of a number of adjustable positions by the bolt 122 which passes through the quadrant slot 123 in support 124 and is screwed into the radial lever portion. The lever 121 has an axial portion 125 having a pointer 126 to accurately set the rotary-position of timing valve 106. The lower timing valve 114 similarly fits bore 130 and is splined to the shaft 127 rotatable in bore 128 and similarly sealed and fixed to a radial portion of control lever 131 which is similarly adjustably positioned by the securing bolt 132 which passes through a quadrant slot 133 in support 135. The lever 131 has also an axial portion 134 with a pointer 136 for setting the adjusted rotary position of valve 114. The support brackets 124 and 135 are secured to the end wall 11 by bolts 138 and have surfaces 139 and 141 against which the levers are clamped by the bolts 122 and 132 to locate the levers 121 and 131. The timing valves 106 and 114 have the same thickness as the valve plate 86 and are splined to shafts 118 and 127 to permit relative axial movement so the face surfaces align with surfaces 87 and 89 on the valve plate 86. The face, which contacts the cylinder block of upper timing valve 106 has a shallow port recess 142, shown in full line, the minimum pressure change position. When the timing valve 106 is in the minimum pressure change position, the cylinder port 92 has minimum rotary pressure change movement, the movement between upper dead'center position 92a and the position providing a connection through the long fluid passage provided by recess 142 to main port 101. During this minimum pressure change movement, the

. cylinder port 92 is disconnected from both main ports and the minimum piston stroke provides a minimum pressure change in a cylinder port 92 and its cylinder. The port recess has a wide end and thus large flow capacity end open to low pressure port 101 at end 103 and anopposite small pointed or apex end close to the cylinder block port 92 in the top dead center position 92a by dot dash line, FIG. 4. The shallow exhaust recess 143, shown in full line, is a relief to a radial portion of the peripheral exhaust passage 96 on the valve plate side facing the cylinder block to reduce the valve area on the side facing the cylinder block acted on by hydraulic pressure to obtain proper balance of the pressures on opposite sides of the rotary valve 106. The pressure in pump port 92 and hydrokinetic and hydrostatic bearing pressure in bearing interface 89-91 acts on an area of the front valve face of valve 106 facing face 91 on the cylinder block, reduced by recess 143, so the hydrostatic bearing pressure in bearing interface 87-88 acting on the full opposite rear valve face area to provide an unbalanced force on valve 106 providing a proper sealing pressure between the front valve face and block bearing face 91. Both port recess 142 and exhaust recess 143 are inherently pressure balanced in a rotary direction so pressure in these ports does not cause any turning moment. The rotary timing valve 106 is rotatable, i.e., 65, to position the port recess 142 and exhaust recess 143 in the dotted line positions 142 and 143. This rotation of valve 106 gradually increases pressure change movement and stroke induced pressure change to a maximum at maximum pressure change position, the point at which the opening of port 92 to low pressure port 101 is not advanced by the timing valve toward the dead center position. The exhaust recess functions the same in all positions or the point at which the fluid passage provided by recess 142 is shortened to zero length as shown.

The lower timing valve 114 has a port recess 146 and an exhaust recess 147 and is similarly rotatable from the minimum pressure change position 146-147, shown in full lines, to the maximum pressure change position, shown in dotted lines 146147. The port recess 146 in minimum pressure change position con nects the high pressure port 107 through low flow capacity apex of recess 146 to cylinder port 92 when the cylinder port is a minimum distance from the lower dead center position 92b of port 92 so there is minimum pressure change movement and pressure change. Turning the timing valve 1l4,increases the pressure change to a maximum in the dotted line maximum pressure change position so there is maximum pressure change before cylinder port 92 opens to the high pressure port 107. The exhaust recess 147 functions like exhaust recess 143.

The hydrostatic unit is further described during pump operation when driven in the direction of the arrows A, FIGS. 1 and 4 with the tilt box in forward angle as shown in FIG. 1 so the upper and lower dead center cylinder port positions 92a and 92b are respectively top and bottom dead center with minimum and maximum cylinder volume. When the port 92 of one cylinder has just passed the trailing edge portion 141 of high pressure main port 107, the top dead center position 92a, the small volume of fluid remaining in the cylinder and port 92 below the piston at its full compression stroke, minimum volume, position is under high pressure. With rotation the piston decompresses or lowers the pressure as the piston moves past the top dead center position and the piston retracts increasing the volume in the cylinder and port and reduces the pressure therein proportional to the amount of pressure change movement past the top dead center position, until cylinder port 92 is connected to low pressure main port 101. In a pump at top dead center the pressure change is a pressure reduction or predecompression so in forward the upper timing valve 106 is between top dead center and the leading end of the adjacent low pressure main port and is a predecompression timing valve. This pressure in the cylinder and its port 92 also decreases due to leakage particularly between the surface seals 89 and 91 on the valve plate and cylinder block proportional to the pressure therein and the time until the port 92 is opened to port 101. With predecompression valve 106 in the minimum predecompression position shown in solid lines in FIG. 4, the predecompression recess 142 in the valve face 106 substantially immediately provides at its apex a small capacity flow connection, or low volume flow connection, through the end portion 103 of the low pressure port 101 to gradually equalize the pressure in port 92 with the pressure in port 101. When valve 106 is rotated toward or to the dotted line position 142 the apex of recess 142 is moved toward or to the initially opening or leading end 103 of port 101. Thus, this rotation, arrow C, of predecompression valve 106 provides increasing pressure change movement of each cylinder and its port 92 past top dead center for increasing decompression to reduce this pressure to the pressure in the low pressure port 101 before cylinder port 92 is connected to low pressure port 101. The number of degrees of movement of the port 92 in a pressure change or predecompression phase is preset in order that the pressure in port 92 substantially equals the pressure in port 101 when they open to each other for fluid communication.

Also with the hydrostatic unit operating as a pump rotating in the direction of arrow A with the tilt box in a forward angle, the lower timing valve 114 functions, as the port 92 moves past bottom dead center, minimum volume, position 92b to high pressure main port 107, to control the amount of pressure increase from the low pressure in the cylinder and port 92 to the high pressure in the high pressure port 107 just before they are connected. In forward the pump lower timing valve, between bottom dead center and the leading end of the high pressure main port 107, is a precompression timing valve. When precompression timing valve 114 is in the minimum precompression position, with the port recess 142 in solid line position, the port 92 moves from the bottom dead center position 921) dotted line, FIG. 4, a minimum distance until port 92 opens through recess 146 to the leading edge of high pressure port 107. When precompression valve 114 is rotated, arrow D, e.g. 65 from the full line position to the dotted line of port recess 146 position, the precompression is increased from a minimum to a maximum. During rotary movement of the cylinder block 58, the piston 54 will move or stroke, compressing the fluid and increasing the pressure of the fluid in the cylinder and port 92 from a minimum to a maximum amount depending on the precompression valve position. Since leakage on compression reduces the pressure rise and the volume of fluid in the cylinder is greater during compression, the degree or distance of precompression travel and stroking is more than the degree or distance of predecompression travel. The precompression valve is set at the position in which the pressure in the cylinder and port 92 is substantially equal to the pressure in the high pressure port 107 so there is no surge of fluid causing noise when these ports are initially connected. If there is any variation from this condition the initial opening has a low flow volume or is restricted to damp any fluid surge for quiet operation.

With the hydrostatic unit acting as a pump with the shaft driven in the same direction A and the tilt box angle reversed from that shown in FIG. 1, the direction of pumping will be reversed, low pressure will enter main port 107 and high pressure will be delivered to main port 101. Then the top and bottom dead centers and the function of the timing valves will be reversed. The timing valve 106 will in reverse function as a precompression valve as described above in relation to timing valve 1 14 and the timing valve 114 will function as a predecompression valve as described above in relation to timing valve 106.

If high pressure is supplied in port 107 and the tilt box angle is as shown in FIG. 1, the motor will be driven in the direction of arrow B and the valve 114 will be a predecompression valve and valve 106 will be a precompression valve. Precompression timing valve 106 is between the trailing end of low pressure port 101 and top dead center cylinder port position 92a and on movement from the above described minimum to maximum positions provides minimum to maximum precompression movement and stroke induced precompression to equalize cylinder port pressure with main port high pressure just as they are interconnected. The predecompression timing valve 114 similarly controls predecompression. When the hydrostatic unit acts as a motor with high pressure supplied to port 101 and the tilt box angle reversed from that shown in FIG. 1, the unit will rotate as a motor driving in the direction of arrow B (FIG. 4), and timing valve 106 will act as a predecompression valve and timing valve 114 will act as a precompression valve as explained in detail above.

From the above it will be seen that in a pump, the timing events, precompression and predecompression occur at the valve main port opening and thus the precompression and predecompression valves are located adjacent the opening or leading end of the main ports. When the hydrostatic unit operates as a motor, the timing events occur at the valve port closing and the timing valves are located adjacent the closing or trailing ends of the main ports. This arrangement of timing valves will function for one direction of rotation for pump operation and the opposite direction of rotation for motor operation. i i

The splined small diameter portion 62 of cylinder block 58 is splined to the splines 151 on the shaft 21. These splines permit a small freedom of movement between the cylinder block and shaft so the block is free to seat on the valve plate. The cylinder block movement away from the valve face is limited by the snap ring 152 but the snap ring is not normally loaded since the biasing device 153 normally biases the cylinder block to contactthe valve plate. The biasing device, shownin FIGS. 1' and 2, has a first annular piston 154 having an outer diameter seal 156 with a cylindrical surface 157 on the inner diameter of the cylinder block and inner diameter seal 158 with an outer diameter surface 159 on the shaft 21. A second piston 161 has similar seals 162 and 163. When fluid is supplied under pressure via the inlet passage 164 in the shaft 21, to the space 166 between the two pistons, the piston 161 abuts shoulder 167 on the shaft and piston 154 engages snap ring 168 in the cylinder block, biasing the cylinder block against the valve plate with a force proportional to the fluid pressure therein. The pistons I54, 161 have respectively an annular ring projection I71 and 172 which have a plurality of recesses 173 at the facing surfaces thereof, to keep the pistons spaced apart so the connection to the inlet passage 164 and governor feed passage 174 is never blocked by the pistons. The governor feed passage 174 extends through the cylinder block between cylinders to the governor valve 176.

The governor valve 176, FIG. 3, has a movable valve element 177 having a stern 178 reciprocably mounted in valve sealing relation in the bore or passage 174 and a head portion 179 cooperating with the sealing edge 181 on a fixed seat member 182 positioned in an enlarged bore portion 183 of the bore 174 with sufficient clearance to provide an exhaust passage between seat member 182 and bore portion 183 to a cross slot passage 184. The seat member is retained therein by snap ring 185 but is free to move laterally in the bore for seating alignment with the valve head 179. The fluid under pressure in passage 174 communicates through a central passage 186 in valve element 177 to the chamber 187 between the valve element head 179 and the seat member 182. When the pressure in chamber 187 overcomes centrifugal force acting on valve element 177 to open the valve, the fluid is permitted to flow to the exhaust space 188 and communicates through the clearance 183 which may be supplemented by axial slots 189 in the perimeter of closure member 182 and cross slot passage 184, around and through the closure member to the space 191 between the cylinder block 58 and housing from which it drains to sump 192 and is evacuated through exhaust passage 193, The fluid level in sump 192 is kept low so the fluid does not contact the rotating cylinder block or other rotating parts. All leakage fluid is drained to the sump 192 as by drain passages 194, 195 which drain control fluid leakage and lubricating fluid via space 191 to sump 192. The internal sump 192 exhaust passage 193 is connected to an external sump 196 or directly to the engine driven pump 197. The engine driven pump 197, at a pressure regulated by the regulator valve 198, supplies fluid under pressure, i.e., 100 psi, to the supply passage 199 which has a branch 200 to supply other requirements, control system, supercharge system and lubrication and communicates via the transfer passage 201 through the restriction 202 in line 164. The transfer passage 201 conveys fluid from supply passage 199 in the fixed housing to passage 164 in the rotary shaft to supply fluid to the space between the pistons 154, 161.

The governor valve 176 regulates the pressure downstream of the restriction 202 at a governor pressure proportional to the speed of rotation of the cylinder block. The restriction 202 limits flow to, and exhaust from, the governor valve so there is constant governor pressure in the passages between the restriction 202 and governor valve 176 to act on pistons 154, 161 to bias the cylinder block against the valve plate with a force proportional to speed. This auxiliary biasing device augments the built in cylinder block balance to prevent cylinder block lift off at high speeds to extend the high speed operating range of the hydrostatic machine.

HYDROSTATIC TRANSMISSION AND VALVE TIMING SYSTEM The hydrostatic transmission, FIGS. 6, 7a and 7b, has a pump 211 and a motor 212 interconnected by hydrostatic power circuit pressure lines 213 and 214 which transmit the high and low pressure hydrostatic fluid between the pump and motor and controls operating as described below. The pump, motor and controls are shown diagrammatically and described from the control viewpoint. The pump and motor sectional views in FIG. 7 are true, corresponding to FIG. 4, but in FIG. 6 they are diagrammatic views showing portions in different planes as shown in FIGS. 1 to 4 and 7. The pump and motor are preferably like the pump of FIGS. 1 to 5 which may be referred to for further structural and operational details. The pump 211 has a fixed housing 216 rotatably supporting input shaft 217 for drive by an engine in the direction of arrow E. Cylinder block 218 and shaft 217 are drive connected by spline drive 219 for relative axial and tilting movement so the block valve surface or face 221 engages and seats on the valve face 222 on the valve plate 223 which like above valve plate 86 is attached to the housing end wall 224 or may be integral with the housing end wall. The cylinder block 218 has an annular series of cylinders 226 each having a port opening 227 extending to cylinder block valve face 221 and alternately connected to the lozenge shaped system main ports 228 and 229 in the valve plate which are respectively continuously connectedto passages 213 and 214.

The upper, as viewed in the drawing, timing or pressure change valve 231 has a port recess 232 and an exhaust recess 233 and is spline connected to control shaft 234. This valve 231 and control shaft 234 are rotatably mounted, axially located and sealed in, respectively, in valve plate 223 and housing end wall 224. The control shaft 234 is fixed to lever 236 which is connected by pin and slot connection to rod 238 of upper valve actuator 129. The actuator 239 controls the rotary position of the timing valve 231 between a minimum position of recess 232, solid line FIG. 7a, and a maximum position of the recess 232' dotted line, FIG. 7a. Each cylinder 226 and its port 227 at the upper dead center 227a, as viewed in the drawing, the position at which it is just disconnected from the previously connected or preceding main port 228, remains disconnected from both main ports during pressure change rotary movement in which the piston stroke movement changes the pressure until cylinder port 227 is initially connected by recess 232 to the subsequent main port 229. The pressure change movement and pressure change due to piston stroke varies from a minimum to a maximum as the timing valve 231 rotates from minimum to maximum position. Upper timing valve 231 controls in forward tilt box angles the stroke effected reduction of pressure in the cylinders, predecompression, between top dead center 227a, minimum cylinder volume position and the leading end of the low pressure main port 229 and in reverse tilt box angles the stroke effected increase of pressure in the cylinders precompression, between the same dead center 227a, now bottom dead center and the leading end of the same main port, now the high pressure main port. The actuator rod 238is fixed at one end to governor piston 241 in governor cylinder 242 in control body 243 fixed to housing 216. Cylinder 242 has pressure change increasing chamber 244 connected to forward speed signal line 356 on one side of the piston and pressure change decreasing chamber 246 connected to reverse speed signal line 357 and spring 247 on the other side of the piston. Rod 238 is fixed at the other end to displacement piston 248 in cylinder 249 in body 243. Cylinder 249 has pressure change decreasing chamber 251 connected to displacement signal 348 on one side and chamber 252 vented by exhaust 253 on the other side. A one-piece stepped piston 254 has a large piston portion a in large bore portion 256 and a small piston portion b in small bore portion 257. System pressure line 214 provides low pressure in forward and high pressure in reverse to step chamber 258 in bore 256 acting on the differential area of piston a, the area of piston a less the area of piston 12, and the high system pressure line 333 provides high pressure in forward and reverse to chamber 259 in bore 257 acting on the area of piston b both acting in a pressure change increasing direction.

Similarly, lower timing valve 261, as viewed in the drawing, has a port recess 262 and exhaust recess 263 and is connected by shaft 264, lever 266 and pin and slot connection 267 to rod 268 of lower valve actuator 269.

This actuator 269 similarly controls timing valve 261 to position the recess 262 in minimum position 262, solid lines, and maximum position 262, dotted lines, FIG. 7a. Each cylinder 226 and its port 227 at lower dead center 227b, the position at which it is just disconnected from the previously connected or preceding main port 229, remains disconnected from both main ports during pressure change movement, in which piston stroke changes pressure therein, until cylinder port 227 is initially connected by recess 262 to the subsequent main port 228. The pressure change movement and pressure change due to piston stroke varies from a minimum to a maximum as timing valve 261 moves from minimum to maximum position. Timing valve 261 similarly controls, in forward operation. the increase of pressure, precompression between bottom dead center and the leading end of the high pressure main port, and in reverse operation the decrease of pressure, predecompression, between the same dead center, now top dead center and the same main port now at low pressure. The rod 268 is secured at one end to governor piston 271 in governor cylinder 272 in control body 273. Cylinder 272 has pressure change increasing chamber 274 connected to reverse speed signal line 357 on one side of the piston and pressure decreasing chamber 276 connected to forward speed signal line 356 and spring 277 on the other side of the piston. Rod 268 has fixed at the other end displacement piston 278 in cylinder 279 having at one end compression decreasing chamber 281 connected to displacement signal line 348 and at the other end chamber 282 vented by exhaust 283. A one-piece stepped piston 284 has a large piston portion a in large bore portion 286 and small piston portion b in small bore portion 287. System pressure line 213, high in forward and low in reverse is connected to the step chamber 288 at the step between the bores and high system pressure line 333, high in forward and reverse is connected to the end chamber 289 so both act to increase pressure change.

Each cylinder 226 has a piston 291 pivotally connected to a slipper 292 which slidably engages the bearing surface 293 on the tilt box 294. The tilt box 294 has a trunion 296 at each side pivotally mounted in the side housing portion 297 to permit tilting of the tilt box from neutral position N through forward displacement anglefand the reverse displacement angle r. The piston reaction forces tend to move the tilt box to the neutral position N. Variable displacement control actuators or motors 298 and 299 vary the tilt box position and thus vary displacement. The reverse control motor 298, at the top of the FIGURE is so called because reverse displacement is increased when a higher pressure is supplied to this motor. The forward motor 299 at the lower part of the FIGURE, is so called because forward displacement is increased when a higher pressure is supplied to this motor. Each motor has a piston reciprocally mounted in a cylinder and connected by a piston rod to the tilt box. A coil spring in each cylinder biases the pistons and the tilt box to neutral position.

The control system has a reservoir or sump 301 from which fluid is supplied by the pump inlet line 302 to the input shaft driven supercharge pump and regulator 303 which supplies fluid at a regulated pressure, i.e., psi, to the main line 304. Main line 304 supplies makeup supercharge pressure respectively through one-way valved branch passages 306 or 307 permitting flow only from main line to the one of the lines 213 or 214 which is under suction or lower pressure (less than 100 psi) and under the higher (above 100 psi) pressure in the other line blocking flow to the high pressure line. The main line 304 also supplies fluid under pressure to the'governor 309 which provides a pump speed governor signal pressure proportional to shaft speed in the speed signal or pump governor pressure line 311. This governor may be a conventional transmission governor driven by shaft 217 or the governor of FIGS. 1 and 3.

The displacement control valve 312, a follow-up type valve, has a valve sleeve element 313 located in a valve bore 314 in valve body 315, a spool valve element 316 having lands a and b or equal diameter is located in the bore 317 of the sleeve 313. The sleeve has an end wall 318 providing a seat for spring 319 positioned between the end wall and land b of valve spool 316. The valve sleeve 313 has an ear 321 connected to control lever 322 pivotally mounted on body 315 by pivot 323 and by link 324 pivoted to the lever and ear to move the valve sleeve 313. The manually operated control level 322 is movable from a central neutral position N in one direction through increasing forward displacement positions to a full displacement forward position 1F, limited by a stop not shown, and in the opposite direction through increasing reverse displacement positions to a full displacement reverse position R, limited by a stop not shown, and similarly positions the valve sleeve 313 and tilt plate 294.

Spool valve element 316 is moved in accordance with displacement by a cam follower rod 326 guided for reciprocal movement in a housing guide portion 327 and having one end engaging a seat 328 in the end of the valve element opposite spring 319 and the other rod end engaging cam 329 fixed on trunion 296 and movable with the tilt box 294 to move the valve element 316 in accordance with displacement of the pump unit 21 1.

The hydrostatic system pressure lines 213 and 214 are respectively connected by one-way valve passages 331 and 332 permitting flow only from the system pressure line having higher pressure to the high system pressure passage 333 and blocking flow to and from the other system pressure line having lower pressure. The system pressure passage is connected to the central port 334 of bore 314 of displacement control valve 312. The upper or reverse port 336 is connected to the reverse control pressure passage 337 and reverse control motor 298. The lower or forward port 338 is connected to the forward control pressure passage 339 and forward control motor 299. The valve sleeve 313 has a central port 341, a reverse port 342 and a forward port 343, each having an annular external recess in the outer diameter of the sleeve and being connected through the sleeve by an annular series of apertures to an internal port opening. The external recesses of ports 341, 342 and 343 in the sleeve are connected respectively to ports 334, 336 and 338 in the valve body in all positions of the sleeve relative to the valve body. The internal port opening of central port 341 is always connected to the space between lands a and b of spool valve 316. The distance between the internal port openings of reverse and forward ports 342 and 343 is the same or slightly larger than the distance between lands a and b so in neutral position, with tolerance latitude, the flow of high system pressure from line 333 to both displacement control passages 337 and 339 is blocked. The lands a and b of spool valve 316 have a small normal clearance in sleeve bore 317 which is sufficient to provide a damped controlled rate of change of displacement readily controlled by clearance and land length. Exhaust 344 in sleeve end wall 318 vents leakage exhaust across land b and the other end of the sleeve is open to vent leakage exhaust across land a.

The reverse and forward pump displacement control lines 337 and 339 are respectively connected by oneway valved passages 346 and 347 to supply the higher displacement control pressure which is proportional to the amount of forward or reverse displacement from neutral, to displacement signal line 348 which is connected to chambers 251 and 281 respectively of predecompression actuator 239 and precompression actuator 269.

A governor signal control valve 351 has a valve element 352 having equal diameter lands a, b, c and d slidable in bore 353. Forward displacement pressure line 339 is connected by branch 354 to chamber 355 at one end of bore 353 to act on the free end of land a to position the valve element as shown connecting pump speed signal line 311 between lands b and c to forward speed signal line 356 and connecting reverse speed signal line 357 between lands a-and b to exhaust 358. Reverse displacement pressure line 337 is connected by branch 359 to chamber 361 at the other end of bore 353 to act on the free end of land d to position valve element 352 to connect speed signal line 311 to reverse speed signal line 357 and connect forward speed signal line 356 to exhaust 362. Theforward speed signal line 356 is connected to increasing chamber 244 of predecompression actuator 239 and decreasing chamber 276 of precompression actuator 269. The reverse speed signal line 357 is connected to decreasing chamber 246 of predecompression actuator 239 and increasing chamber 274 of precompression actuator 269.

The motor 212 is similar or structurally the same as the pump or the hydrostatic unit of FIGS. 1 to 5. The motor has a stationary housing 366 with suitable bearings rotatably supporting output shaft 367 and cylinder block 368 which is drive connected by spline drive 369 to the shaft. The block has an annular valve face 371 seated on the annular valve plate face 372 of valve plate 373 seated on end wall 374. The block has an annular series of cylinders 376 each having a port 377 connected during rotation alternately to port 378 and pressure line 213 and then to port 379 and pressure line 214.

In the motor the timing valves 381, 411 have the same structure as the pump timing valves and function similarly. Since these timing valves only function during rotation of the hydrostatic unit in one direction, the motor timing valves are constructed and described below with reference to forward motor drive, arrow H. Since the motor tilt box always remains in a forward drive angle and the motor drive is reversed by reversing the pump tilt box angle so the pump. though driven in the same direction, reverses the high and low system pressures supplied to the motor, to provide reverse motor drive, the pump timing valves function during both forward and reverse drive and the motor timing valves only function during forward drive. The upper precompression valve 381 as viewed in the drawing, or, located near top dead center or minimum cylinder volume position has port recess 382 and exhaust recess 383 and is splined to and rotates respectively in the valve plate and housing with control shaft 384. This shaft is connected by lever 386 and pin and slot connection 387 to rod 388 of precompression actuator 389. The rod has, fixed on one end, governor piston 391 slidable in cylinder 392 in actuator body 393 fixed to the motor housing 366. The piston divides the cylinder into chamber 394 vented by exhaust 395 and chamber 396 connected to governor line 452 and spring 397 both for decreasing precompression. Rod 388 has fixed at the other end displacement piston 398 sliding in cylinder 399 and dividing the cylinder into chamber 401 connected to displacement signal line 348 for decreasing precompression and chamber 402 vented by exhaust 403. Stepped piston 404 has a large land a in large bore 406 and a small land b in small bore 407. The high forward pressure in line 213 is connected to chamber 409 and in branch line 410 to step chamber 408 to act to increase precompression. The motor and pump timing valve actuators have the same structure. In the motor actuators, single area pistons with the precompression actuator piston having a larger area than the decompression actuator piston could replace the respective stepped pistons 406 and 434.

In the motor the lower or predecompression timing valve 411, located near bottom dead center, has a port recess 412 and exhaust recess 413 and is splined to control shaft 414 connected by lever 416 and pin and slot connection 417 to rod 418 of motor predecompression actuator 419. The rod 418 has, fixed on one end, governor piston 421 slidable in cylinder 422 in body 423 fixed to motor housing 366. Piston 421 divides cylinder 422 into increasing displacement chamber 424 connected to governor line 452 and chamber 426 vented by exhaust 425. The spring 427 in chamber 426 decreases precompression. Rod 418 at the other end has fixed thereto displacement piston 428 sliding in cylinder 429 and dividing the cylinder into chamber 431 connected to displacement signal line 348 for decreasing predecompression and chamber 432 vented by exhaust 433. The stepped piston 434 has large land a in large bore 436 and small land b in small bore 437 providing step chamber 438 vented by exhaust 440 and end chamber 439 connected to forward high system pressure line 213 for increasing precompression.

In the motor cylinder block 368 each cylinder 376 has a piston 441 pivoted to a slipper 442 contacting bearing surface 443 for connecting the pistons to tilt box 444. The tilt box 444 is rotatably supported by trunions 446 at each side rotatably mounted in opposite housing side wall portions 447 and positionied by actuators 448 and 449 in the range of'forward angular positions f between neutral N and forward F.

The motor displacement control valve 312 is the same as the pump displacement control valve 312 so the same reference-numerals primed have been used and the drawing simplified. The controllever 322 is movable between neutral N and drive position D, limited by stops not shown. The valve 312 is responsive to tilt box position as signalled by cam 329' on tilt box 444 and the position of lever 322 to control the supply of fluid to drive displacement line 337 to motor 449 to control tilt box angle in accordance with the position of lever 322'.

The motor governor 451, constructed like the pump governor 309, is operatively connected to motor or output shaft 367 and supplied with fluid from main line 304 to provide a motor speed signal pressure in lin 452 proportional to motor speed.

OPERATION The transmission shown in FIGS. 6, 7a and 7b in the forward drive operating phase with the pump shaft driven in the direction of arrow E and the tilt box 294 in a forward drive displacement angel, position F, as shown in FIG. 6, delivers hydrostatic system high pressure fluid to pump port 228, dotted line in FIG. 6 as it is behind the central section, connected to system line 213 which flows as indicated by the arrow FP to the motor port 378, dot dash lines in FIG. 6 as it is in front of the central section of motor212 to drive its output shaft 367, in the direction indicated by arrowH. The motor exhaust fluid flows through motor port 379, dotted line in FIG. 6 as it is behind the central section, and is conveyed by the low pressure return line 214 as indicated by the arrow F5, to the port 229, dot dash line in FIG. 6 as it is in front of the central section. In forward drive the pump port 228 is the high pressure outlet or supply port supplying high system pressure supply line 213 and high pressure motor inlet port 378. The motor outlet or return port 379 returns fluid 'at low pressure through low pressure return line to the motor inlet or suction port 229.

The supercharge pump and regulator 303 and main line 304 supplies regulated main line pressure through one-way valve 307 to supply makeup and supercharge pressure fluid to the low pressure power system line 214. The high power system pressure in line 213 closes one-way valve 306 and opens one-way valve 331 to supply high power system pressure to line 333 to provide the high power system pressure signal and to close one-way valve 332. Main line pressure is also connected to the pump and motor governors 309 and 451 which respectively, during rotation of pump shaft 217 and motor shaft 367, provides a pump and motor speed signal pressure in pump and motor governor signal lines 311 and 452.

In order to maintain the pump 211 with tilt box 294 in the full displacement position F, the displacement control lever 322 is placed in full forward position F as shown in FIG. 6 and the sleeve 313 and the spool valve 316 are in forward,position. High system pressure line 333 is always connected by central ports 334 and 341 to bore 317 between lands a and b of spool 316. In this maintaining position-the spool316 is slightly out of the central position with respect to the sleeve so that the land a very slightly closes the opening of port 342 so a very low pressure is supplied to reverse passage 337 and motor 298 and so. the'land b slightly opens the openings of port 343 so that higher pressure is supplied to the. forward control line. 339 and motor 299. The forces provided by these pressures and the motor springs balance the reaction forces of the tilt box 294 to hold the tilt box in the full displacement position. Makeup flow for the slightly higher leakage in the motor 299 due tothe higher pressure therein as compared to motor 298 is also provided. These conditions, to a reduced degree, will exist in other intermediate displacement positions. When it is desired to reduce displacement, the lever 322 is moved from the F position toward the N position, initially causing the sleeve 313 to move relative to the spool to provide a partial or full connection from high system pressure line 333 through ports 334, 341,the space between the lands, ports 342, 336 and to line 337 and motor 298 to reduce displacement. Fluid pressure is exhausted from motor 299 through line 339, port 338 and through the clearance space around land and across land b. This occurs at a controlled -predetermined rate to control the rate of decreasing displacement. I

As the pressure in motor 298 reduces displacement, the tilt box rotates and rotates cam 329 pushing on rod 326 to move the valve spool 3l6against spring 319 in a closing direction to close the valve or return the valve to a partial displacement maintaining position, like the above described full displacement maintaining position when the tilt box reaches the lower angle called for by the position of the displacement control lever 322 to terminate control movement and maintain the desired displacement position.-:

In a similar manner, when the control lever 322 is moved from neutral or a low displacement position to a higher displacement position in'the forward range, the forward angle of the tilt box 294 is increased and maintained at the position called for by the displacement control lever.

Positioning the displacement control lever 322 in reverse r positions between N and R similarly positions the tilt box 294 in reverse angle r positions which reverses the direction ofthe pumping action and supplies high pressure fluid fromthe pump to the motor via systern line 214 and then the line 213 acts as a return line returning the fluid'at'low pressure to the pump intake. During vehicle overrun, the displacement control will be operated in the forward quadrant since the motor 212 on'overrun acting as a pump supplies high pressure fluid via line 214fi'n the direction of the arrow F3 to the pump 211 which now acts as a motor and returns fluid at low pressure in the direction of arrow FP through line 213 to the motor 212.

Since the valve spool 316 has a clearance in the sleeve bore 317, there is restricted supply flow from inner supply port 341 and the space between lands 316a and b to inner supply ports 342, 343, which is less restricted'than-the exhaust now; to pressurize both motors. Movement of the tilt'b'ox 294moves the spool valve relative to the sleeve to rapidly decrease the supply restriction and slightly increase the exhaust restriction of one motor and increase the supply restriction and decrease the exhaust restriction of the other motor to move the tilt box in the opposite direction. Thus each maintaining coinciding position is an auto regulating position, regulating equal pressures in the motors for neutral and increasingpressure in motor 299 for increasing forward displacement and increasing pressure in motor 298 for increasing reverse displacement and responsive to tilt box drift from any position to so control flow to and exhaust from the motors to counteract the drift for auto regulation of displacement at any controlled sleeve member position.

The motor displacement control lever 322' is movable from a neutral position N through increasing drive displacement positions to full displacement drive position D to control motor displacement control valve 312 to control the displacement position of motor'tilt box 444 like the above described pump displacement control valve 312 and tilt box 294. Various modes of joint and differential operation of the pump control lever 322 and the motor control lever 322' may-be used. The pump is placed in neutral position for transmission neutral and displacement increased to reduce torque multiplication of the transmission. The motor is normally movable from a low to full displacement position to increase torque multiplication of the transmission. The pump timing valves 231, 261 are automatically controlled by actuators 239, 269 in response to pump speed, displacement and power system pressure to control the pressure in each cylinder and its port 227 so it equals the pressure in each main port 228, 229' at the time they are initially interconnected so there is no pressure surge. Each cylinder and port 227, at the moment it is disconnected from one main port at the dead center position contains a pressure equal to that one main port pressure. During continued dead center zone movement or pressure change movement of the cylinder and port 227, while disconnected from both main ports, piston movement or stroke changes the cylinder and port 227 volume to change the pressure toward the pressure in the other main port. The timing valves 231, 261 are movable from a minimum to a maximum position providing minimum to maximum pressure change movement and pressure change until port 227 is initially connected to the other main port. The pump when driven in the direction of rotation, arrow B, with tilt box 294 in a forward drive angle position delivers high pressure fluid to port 228 and receives lowpressure fluid from port 229. The top dead center timing valve functions as a predecompression timing valve as it controls the amount of stroke effected reduction of pressure in the cylinders and ports 227 as they move past the high pressure port 228 from top dead center 227a toward the low pressure port 229. The bottom dead center timing valve functions as a precompression timing valve as it controls the amount of stroke effected increase of pressure in the cylinders and ports 227 as they move past the low pressure port 229 from bottom dead center 2271) toward the high prssure port 228.

The amount of precompression or predecompression required to equalize the pressures in cylinders and ports 227 and the system main ports at the instant each cylinder port 227 is intially connected to a main port depends on the following operating conditions. Increased precompression and predecompression is required to compensate for increased fluid Compression caused by higher system pressureand greater fluid cylinder volume. Leakage is increased by higher presssure, speed, temperature, parts clearance and lower viscosity. increased leakage requires increased precompression and decreased decompression for compensation. More precompression or decompression is needed at bottom dead center than at top dead center because the fluid volume is greater at bottom dead center. As the displacement or stroke of a variable displacement axial piston pump or motor is reduced, the cylinder volume at top dead center increases and the volume at bottom dead center decreases and the linear piston travel is reduced relative to shaft rotation.

In forward pump operation, upper actuator 239 acting as a predecompression actuator is basically controlled from minimum to maximum positions by the decompression decreasing bias of spring 247 and increasing bias of high forward system pressure from system line 213 and high system pressure line 333 in chamber 259 acting on land 2541?. The lower actuator 269 acting as a precompression actuator is basically controlled from minimum to maximum positions in the same way by the compression decreasing bias of spring 277 and increasing bias of forward high system pressure from high system pressure line 333 acting in chamber 289 on land 2841). Leakage and larger cylinder volume at bottom dead center requires more precompression than predecompression. The high system pressure line 213 is connected to chamber 288 to act on land 284a to provide a large correction increase of precompression proportional to high system pressure. The low or supercharge system pressure line 214 is connected to cham her 258 and acts on land 254a and does cause a relatively small predecompression increasing force that can be balanced by the spring and thus is not required in forward but is required in reverse as explained below. Since with increasing displacement less predecompression and precompression is required, the displacement signal line 348 with a pressure proportional to displacement'is connected to chamber 251 and chamber 281 respectively of the predecompression actuator 239 and precompression actuator 269 to reduce predecompression and precompression. Since with increasing pump speed more-predecompression is required, the forward governor signal line 356 is connected to chamber 244 of the predecompression actuator. Also with increasing 1 pump speed less precompression is required so the forward governor signal line 356 is connected to chamber 276 to decrease precompression. This speed correction is a minor correction, particularly in hydrostatic units having'speed responsive biasing means.

For reverse operation the tilt box is placed in a reverse angle position and the same pump rotation continues, arrow E. The high and low pressures in the main ports are reversed so high pressure is provided in main port 229 and system line 214 and low pressure in main port 228 and system line 213. Alsotop and bottom dead centers are reversed so upper, as viewed in the drawing, dead center 227u'is bottom, maximum cylin der volume, dead center and lower dead center is top dead center. This reverses the compression and decompression functions ofthe timing valves so upper timing valve23l is a precompression timing valve and lower timing valve 261 is a predecompression timing valve moving in the same way from minimum to maximum positions. The functions of the upper and lower actuators 239 and 269 are also reversed providing precompression actuator 239 and predecompression actuator 269. The basic control bias remains the same since system line 213 now supplies high system pressure through high system pressure signal line 333 to chambers 259 and 289 to respectively act on lands 25417 and 284!) to increase precompression and predecompression against the opposing bias of springs 247 and 277. To provide the correction increasing precompression relative to predecompression system line 214 now is at, and supplies, high system pressure to chamber 258 to act on land 254a to increase precompression by a larger amount than the increase in predecompression pro vided by low system pressure now provided by system line 213 to chamber 288 to act on land 284a. The dis placement signal line 348 supplies a pressure proportional to reverse displacement to chambers 251 and 281 respectively of upper and lower actuators 239 and 269 to decrease precompression and predecompression in the same way. In reverse shuttle valve 361 connects pump governor line 311 to reverse governor line 357 connected to chambers 246 and 274 respectively decreasing precompression and increasing predecompression.

In forward drive operation, the pump 211 supplies high system pressure by system line 213 to motor main port 378 and motor main port 379 returns low system pressure by system line 214 causing the motor 212, when the tilt box 444 is in a drive angle position D so upper dead center 377a is top dead center, to rotate so the cylinder port 377 moves from low to high pressure main ports at top dead center, the forward drive rotation, arrow H. Thus motor rotation of the cylinder block relative to the valve plate is opposite to pump rotation. Since the pump and motor are arranged back to back, pump shaft 217 and motor shaft 367 rotate in the same direction relative to the vehicle. In reverse drive operation, the pump 211 supplies high system pressure by the other system line 214 to motor main port 379 and motor main port 378 returns low system pressure by system line 213 to the pump, causing the motor, when the tilt box 444 is in a drive angle position D, to rotate in the opposite direction to arrow H for reverse drive.

In the motor during forward drive, the upper timing valve 381 is a precompression timing valve and the lower timing valve 411 is a predecompression timing valve respectively controlled by actuators 389 and 419. Since the timing valves do not function during reverse drive only the connections necessary for forward drive are used. Both actuators 389 and 419 are basically controlled by high system pressure supplied by line 213 respectively to chambers 409 and 439 to act on lands 404b and 434b to provide a precompression and predecompression increasing bias opposed by the bias of springs 397 and 427. The forward or drive displacement pressure line 337 is connected to chambers 401 and 431 respectively of actuators 389 and 419 to provide a correction bias decreasing both precompression and predecompression with increasing displacement. The motor governor signal line 452 is connected to chamber 396 of precompression actuator 389 to provide a correction bias descreasing precompression with increasing speed and also connected to chamber 424 of predecompression actuator 419 to provide a correction bias increasing predecompression with increasing speed. As in the pump, particularly if a speed controlled means to bias the cylinder block against the valve plate is used, the speed correction may not be needed.

in reverse drive the timing valves are not functional and the actuators position both timing valves in their minimum position. Since system line 213 is at low pressure, the spring bias and displacement bias act to place the actuators in minimum position regardless of the pressure of the lower governor bias on actuator 419.

MODIFIED TIMING VALVE The modified timing valve 456 a rod 457 mounted for reciprocation in bore 458 in valve plate 86' which is otherwise like the above valve plate 86. Primed reference numerals have been applied to the main basic similar parts. A plurality of apertures 459 extend from the valve plate surface 89 to bore 458. The bore extends from the outer perimeter of valve plate 86' to port 101'. The valve rod 457 is shown in the minimum pressure change position with end 461 further from the main port 101' than apertures 459 and is movable to maximum pressure change position, placing end 461 at 460 closing all or substantially all the apertures 459. The apertures 459 have a small area and flow capacity compared to the area and flow capacity of the bore 458 so the flow capacity of the connection provided between the cylinder port 929' and the main port 101' is gradually changed in a manner similar to the gradual capacity change of this connection provided by the apex shape of the recess 142 of rotary timing valve 106. This gradual change of flow capacity takes place during the opening or closing of the 3 to 5 of the apertures 459 next adjacent the reciprocal timing valve rod end 461 in any valve position and thus is substantially uniform in all valve positions. The bore 458 with apertures 459 may extend beyond the maximum pressure change position 460 of the valve rod end so there is a gradual pressure change interconnnection in all timing valve positions.

The actuator 462 has a rod 463 connected to valve rod 457 so they move together. Since at .least supercharge pressure is always present in port 101', the connection may be a mere contact but other connections including a bell crank may be used if it is desired to place the actuator in an axial position. The actuator rod 463 is secured to a governor piston 464 in cylinder 465 in a valve body 466. The piston 464 divides the cylinder into a pressure change decreasing chamber 467 supplied with a governor pressure by govenor signal line 468 and pressure change increasing chamber 471 sup? plied by governor signal line 472. The actuator rod 463 is also secured to displacement piston 473 mounted for reciprocation in cylinder 474 and dividing the cylinder into pressure change decreasing chamber 476 connected to displacement signal line 477 and chamber 478 vented by exhaust 479. Spring 481 in chamber 476 also acts to decrease pressure change. The step piston 482 has a large land a in alarge bore 483 and a small land b in small bore 484. A high or low system pressure line 486 is connected to chamber 487 at the step between the bores to act on the area ofland a less the area of land b to increase the pressure change bias. The high pressure line 488 is connected to chamber 489 to act on the area of land a to bias the actuator for increased pressure change. Since the pressure in main port 101 exerts a bias on the end 461 of valve rod 457 in all positions, the area of land 482a is increased by the area of end 461 as compared to the above actuators, i.e., 239.

Then since port 101' is connected in the system to line 486 and chamber 487 to act on the differential area of land 482a, the pressure on end 461 is balanced by the same pressure on part ofland 482a and this pressure on the remaining portion of land 482a provides a pressure change increasing bias. This actuator 462 and timing valve 456 assembly is functionally like the above pump actuators 239, 269 and motor actuators 389 and 419 and their respective timing valves. Thus a valve plate 86 with two reciprocal timing valves 456 each controlled by an actuator 462 may be used in the pump and motor of the hydrostatic transmission shown in FIGS. 6, 7a and 7b. In substituting a reciprocating timing valve 456 and actuator 462, for example, for rotary timing valve 231 and actuator 239 it will be apparent that the valve plate 89' is used with timing valve 456 and that the following system and signal lines of FIG. 7a are connected to lines of actuator 462; high system pressure line 333 is connected to the similar line 488, system line 214 is connected to line 486, displacement signal line 348 is connected to displacement signal line 477, forward governor signal line 356 is connected to governor pressure line 472 and reverse governor pressure line 357 is connected to governor line 468 so each of these pressures provide the same increasing or decreasing pressure change bias. Other timing valves'456 and actuators 462 may be similarly substituted for the other timing valves and actuators in the FIG. 7 transmission to provide a transmission operating in the same manner except for the above described detailed differences specific to the reciprocating timing valve 456 and its actuator 462. As pointed out above, the governor correction of automatic timing valve control is the minimum amount correction and in hydrostatic units where leakage has low variations with speed, the governor piston 464 and cylinder 465 may be omitted.

The lower timing valve 458 and actuator 462to have been shown to indicate their same position relative to the same direction of rotation and the same parts have the same reference numerals primed.

The apertures 459 are overlapped so in each position of valve 457 a similar increment of valve movement provides the same degree of change of pressure change movement so both the reciprocating and the rotary timing valves uniformly determine pressure change movement in response to their position determined by the controls. The apertures 459 have a small area, are very shallow and are sealed by the valve so the reciprocating valves provide an essentially planar pressure change surface like the rotary timing valves.

TIMING VALVE FUNCTION The timing valves are located between a dead center position 227a and 22712 and a spaced adjacent main port 229 and 228 relative to the same direction of rotation. The pump rotates in this direction for forward and reverse pumping, locating the timing valves at the leading edge of the adjacent main port. The motor rotates in the opposite direction for forward drive, locating the timing valve at the trailing edge of the adjacent main port. The annular planar valve surface between each dead center and the spaced adjacent main port may be called a pressure change portion, as a varying part thereof is used to change cylinder pressure, or a transition portion divided by the timing valve into a variable pressure change portion, the variablepart over which cylinder pressure varies, and a main port extension portion, the variable part over which the cylinder and main port are connected and thus at the same pressure. As each cylinder port is at a dead center position and, after in a pump or before in a motor, moves through the pressure change portion with the cylinder port closed to change cylinder pressure. The timing controls control the amount of such movement, in a pump at top dead center decompression pressure change and at bottom dead center compression pressure change and in a motor due to the opposite direction of rotation the opposite. Greater compression and decompression movement is required to compensate for compressability of fluid with increasing high main port pressure and fluid volume in the cylinder to provide the same pressure change. More compression movement than decompression movement is required due to leakage, which increases as a function of power system pressure and decreases as a function of speed, to provide the same pressure change. The cylinder volume is greater at bottom dead center than top dead center so more compression movement at bottom dead center than at top dead center is required for the same pressure change. As displacement increases from neutral, where top and bottom dead center cylinder volumes are the same, the cylinder volume at top dead center decreases and at bottom dead center increases.

The amount of pressure change movement provided by each timing valve is basically controlled by the high main port pressure increasing pressure change movement, i.e., line 333, opposed by a spring. Since more compression movement is required, high main port pressure acts on a larger area or on a second area, i.e., chamber 288 for forward pump operation, on the timing valve controlling compression. As displacement increases the compression and decompression pressure change movement is slightly reduced to correct the main high pressure control with changing cylinder volume due to changing displacement. Also as speed increases compression movement is slightly decreased and the decompression movement is slightly increased to correct for leakage variation with speed.

In the pump and motor high, main port pressure acts on a large area to increase compression movement and acts on a small area to increase decompression movement which is corrected by a smallreduction with increasing displacement and a smaller reduction of compression and increase of decompression with increasing speed. 1

In this specification, reference to the location or position of certain parts as upper or'lower, etc. relative to their position in the drawing, has been made only for convenience in referring to the drawing in conjunction with the specification and are thus merely descriptive of the illustrated preferred arrangements. The specific position of hydrostatic pump and motor units and their components, as is well known in the art, is generally subject to variation and the units may be employed or operated in any position and the components only require the above described relative arrangement for accomplishing the described functions.

It will be appreciated that other equivalent embodiments of the several disclosed embodiments of modifications may be made.

It is claimed:

1. In a hydrostatic pump or motor unit; hydrostatic displacement means including stator means having cam means and rotor means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead center position at top and bottom dead center positions of said rotor means during rotation of said rotor means; said stator means having a planar annular valve face having adjacently arranged in annular sequence in one rotary direction a first dead center closure surface portion. a first transition surface portion having a first timing valve means, a first main port portion, a second dead center closure portion, a second transition surface portion having a second timing valve means and a second main port portion with each portion having one end adjacent the preceding portion and the other end adjacent the following portion; said rotor means having an annular port face in surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylin- I der port being connected to a cylinder and traversing said valve face portions during rotation in said one direction in the same sequence and during rotation in the opposite direction in the opposite sequence and being closed when in full registry at each dead center surface portion at the same point of said rotary movement and open at each main port portion; each timing valve means providing a progressively variable length pas sage between a variable transition point in each transition surface and the one end of the adjacent main port portion dividing the transition surface portion into an essentially planar progressively variable length pressure change surface portion extending to the other end of the adjacent dead center portion and an inversely progressively variable length port extension portion 'extending to the one end of the adjacent main port portion operative to continuously close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change and to continuously 'open each cylinder port to said adjacent main port portion during cylinder port movement over said port extension'portion and said timing valve means progressively movable to a minimum position to lengthen said passage and move said transition point toward said adjacent dead center portion for minimum pressure change movement and pressure change and to maximum position toshorten said passage and move said transition point toward said adjacent main port portion for maximum pressure change movement and pressure change and control means operatively connected to said timing valve means to control said timing valve means to control the amount of pressure change to make the cylinder pressure substantially equal to main port pressure at the transition point for equalized pressure interconnection.

2. In a hydrostatic pump or motor unit; hydrostatic displacement means including stator means having cam means and rotor means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead center position at top and bottom dead center positions of said rotor means during rotation of said rotor means; said stator means having a planar annular valve face having adjacently arranged in annular sequence in one rotary direction a first dead center closure surface portion, a first transition surface portion having a first timing valve means, a first main port portion, a second dead center closure portion, a second transition surface portion having a second timing valve means and a second main port portion with each portion having one end adjacent the preceding portion and the other end adjacent the following pressure at the transition portion; said rotor means having an annular port face in surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylinder port being connected to a cylinder and traversing said valve face portions during rotation in said one di' rection in the same sequence and during rotation in the opposite direction in'the opposite sequence and being closed'when in full registry at each dead center surface portion at the'same point of said rotary movement and open at each main port portion; each timing valve means providing a progressively variable length passage 'betweena' variable transition point in each transition surface and the one end of the adjacent main port portion dividing the transition surface portion into a progressively variable length pressure change surface portion extending to the other end of the adjacent dead center portion and an inversely progressively variable length port extension portion extending to the one end of the adjacent main port portion operative to continuously close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change and to contin'uously open each cylinder port to said adjacent main port portion during cylinder port movement over said port extension portion and said timing valve means progressively movable to a minimum position to lengthen said passage and move said transition point toward said adjacent dead center portion for minimum pressure change movement and pressure change and to maximum position to shorten said passage and move said transition point toward said adjacent main port portion for maximum pressurechange movement and pressure change and'control means operatively connected to each timing valve means to independently control each of said timing valve means to independently control the amount of pressure change to provide a larger length of compression than decompression change to make the cylinder pressure substantially equal to main port point for equalized pressure interconnection.

3. In a hydrostatic pump'or motor unit; hydrostatic displacement means including stator means having cam means and rotor means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead centerposition at top and bottom dead center positions of said rotor means during rotation of said rotor means; said stator means having a planar annular valve face having adjacently arranged in annular sequencein one rotary direction a first dead center closure surface portion, a first transition surface portion having a first timing valve means, a first port portion, a second dead center closure portion, a second transition surface portion having a second timing valvemeans and a second main port portion with each portion having one end adjacent the preceding portion and the other end adjacent the following portion; said rotor means having an annular port face in surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylinder port being connected to a cylinder and traversing said valve face portions during rotation in said one direction in the same sequence and during rotation in the opposite direction in the opposite sequence and being closed when in full registry at each dead center surface portion at the same point of said rotary movement, controlling compression pressure change in one transition portion and decompression pressure change in the other transition portion and open at each main port portion, one having high pressure and the other having low pressure; each timing valve means providing a progressively variable length passage between a variable transition point in each transition surface and the one end of the adjacent main port portion dividing the transition surface portion into a pressure change surface portion extending to the other end of the adjacent dead center portion and a port extension portion extending to the one end of the adjacent main port portion operative to close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change and to open each cylinder port to said adjacent main port portion during cylinder port movement over said port extension portion and movable to a' minimum position to progressively lengthen said passage and move said transition point toward said adjacent dead center portion for minimum pressure change movement and pressure change and to maximum position to progressively shorten said passage and more said transition point toward said adjacent main port portion for maximum pressure change movement and pressure change to variably control the amount of pressure change and control means connected to the main port at high pressure and each of said timing valve means to indepen-. dently control each of said timing valve means to-control the amount of pressure change to provide a larger compression than decompression pressure change movement as a function of main port high pressure to make the cylinder pressure substantially equal to main port pressure at the transition point for equalized pressure interconnection.

4. In a hydrostatic pump or motor unit; hydrostatic displacement means including stator means having cam means and rotor means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead center position at top and bottom dead center positions of said rotor means during rotation of said rotor meansydisplacement control means operatively connected to said cam means to vary the displacement of said hydrostatic displacement means from neutral; said stator means having a planar annular valve face having structurally arranged in annular sequence in one direction a first dead center closure surface portion, a first transition surface-portion having a first timing valve means, a first main port portion, a second dead center closure portion, a second transition surface portion having a second timing valve means and a second main port portion; said rotor means having an annular portfacein surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylinder port being connected to a cylinder and traversing said valve face portions during rotation in said one direction in the same sequence and during rotation in the opposite direction in the opposite sequence and being closed at each dead center surface portion, controlling compression 'pressure change in one transition portion and decompression pressure change in the other transition portion and open at each main port portion one having high pressure and the other having low pressure; each timing valve means providing a variable length passage between a variable transition point in each transition surface and the adjacent main port portion dividing the transition surface portion into a pressure change surface portion extending to the adjacent dead center portion'and a port extension portion extending to the adjacent main port portion operative to continuously close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change and to continuously open each cylinder port to said adjacent main port por tion during cylinder port movement over said port extension portion and movable to a minimum position to lengthen said passage and move said transition point toward said adjacent dead center portion for minimum pressure change movement and pressure change and to maximum position to shorten said passage'and move said-transition point toward said adjacent main port portion for maximum pressure change movement and pressurechange to variablycontrol the amount of pressure change movement and control means connected to the main port at high pressure, said timing valve means, said displacement control means and said rotor means to independently control said timing valve means to control the amount of pressure change to provide a larger compression than decompression pressure change movement as a function of main port high pressure, reduce both as a function of displacement variation from neutral, to reduce compression movement as a function of rotor speed and increase decompression movement as a function of rotor speed to make the cylinder pressure substantially equal to main port pressure at the transition point for equalized pressure interconnection.

- 5. In a hydrostatic pump or motor unit; hydrostatic displacement means including stator means having cam means and rotor'means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead center position at top and bottom dead center positions of said rotor means during rotation of said rotor means; displacement control means operatively connected to said cam means to A provide neutral zero displacement and forward and reverse ranges of displacement; said stator means having a planar annular valve face having structurally arranged in annular adjacent sequence in one direction a first deadcenter closure surface portion, a first transition surface portion having a first timing valve means, a first main port portion, a second dead center closure portion, a second transition surface portion having a second timing valve means and a second main port portion; said rotor means having an annular port face in surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylinder port being connected to a cylinder and traversing said valve face portions during rotation in said one direction in the same sequence and being closed at each dead center surface portion, controlling compression or deicompresssion pressure change in forward or reverse displacement'in each transition portion and open at each main port portion each having low or high pressure in forward or reverse; each timing valve means providing a variable length passage between a variable transition point in each transition surface and the adjacent main port portion dividing the transition surface portion'into a pressure change surface portion extending to the adjacent dead center portion and a port extensionportion extending to the adjacent main port portion operative to-close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change-and to open each cylinder port to said adjacent 

1. In a hydrostatic pump or motor unit; hydrostatic displacement means including stator means having cam means and rotor means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead center position at top and bottom dead center positions of said rotor means during rotation of said rotor means; said stator means having a planar annular valve face having adjacently arranged in annular sequence in one rotary direction a first dead center closure surface portion, a first transition surface portion having a first timing valve means, a first main port portion, a second dead center closure portion, a second transition surface portion having a second timing valve means and a second main port portion with each portion having one end adjacent the preceding portion and the other end adjacent the following portion; said rotor means having an annular port face in surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylinder port being connected to a cylinder and traversing said valve face portions during rotation in said one direction in the same sequence and during rotation in the opposite direction in the opposite sequence and being closed when in full registry at each dead center surface portion at the same point of said rotary movement and open at each main port portion; each timing valve means providing a progressively variable length passage between a variable transition point in each transition surface and the one end of the adjacent main port portion dividing the transition surface portion into an essentially planar progressively variable length pressure change surface portion extending to the other end of the adjacent dead center portion and an inversely progressively variable length port extension portion extending to the one end of the adjacent main port portion operative to continuously close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change and to continuously open each cylinder port to said adjacent main port portion during cylinder port movement over said port extension portion and said timing valve means progressively movable to a minimum position to lengthen said passage and move said transition point toward said adjacent dead center portion for minimum pressure change movement and pressure change and to maximum position to shorten said passage and move said transition point toward said adjacent main port portion for maximum pressure change movement and pressure change and control means operatively connected to said timing valve means to control said timing valve means to control the amount of pressure change to make the cylinder pressure substantially equal to main port pressure at the transition point for equalized pressure interconnection.
 2. In a hydrostatic pump or motor unit; hydrostatic displacement means including stator means having cam means and rotor means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead center position at top and bottom dead center positions of said rotor means during rotation of said rotor means; said stator means having a planar annular valve face having adjacently arranged in annular sequence in one rotary direction a first dead center closure surface portion, a first transition surface portion having a first timing valve means, a first main port portion, a second dead center closure portion, a second transition surface portion having a second timing valve means and a second main port portion with each portion having one end adjacent the preceding portion and the other end adjacent the following portion; said rotor means having an annular port face in surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylinder port being connected to a cylinder and traVersing said valve face portions during rotation in said one direction in the same sequence and during rotation in the opposite direction in the opposite sequence and being closed when in full registry at each dead center surface portion at the same point of said rotary movement and open at each main port portion; each timing valve means providing a progressively variable length passage between a variable transition point in each transition surface and the one end of the adjacent main port portion dividing the transition surface portion into a progressively variable length pressure change surface portion extending to the other end of the adjacent dead center portion and an inversely progressively variable length port extension portion extending to the one end of the adjacent main port portion operative to continuously close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change and to continuously open each cylinder port to said adjacent main port portion during cylinder port movement over said port extension portion and said timing valve means progressively movable to a minimum position to lengthen said passage and move said transition point toward said adjacent dead center portion for minimum pressure change movement and pressure change and to maximum position to shorten said passage and move said transition point toward said adjacent main port portion for maximum pressure change movement and pressure change and control means operatively connected to each timing valve means to independently control each of said timing valve means to independently control the amount of pressure change to provide a larger length of compression than decompression change to make the cylinder pressure substantially equal to main port pressure at the transition point for equalized pressure interconnection.
 3. In a hydrostatic pump or motor unit; hydrostatic displacement means including stator means having cam means and rotor means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead center position at top and bottom dead center positions of said rotor means during rotation of said rotor means; said stator means having a planar annular valve face having adjacently arranged in annular sequence in one rotary direction a first dead center closure surface portion, a first transition surface portion having a first timing valve means, a first port portion, a second dead center closure portion, a second transition surface portion having a second timing valve means and a second main port portion with each portion having one end adjacent the preceding portion and the other end adjacent the following portion; said rotor means having an annular port face in surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylinder port being connected to a cylinder and traversing said valve face portions during rotation in said one direction in the same sequence and during rotation in the opposite direction in the opposite sequence and being closed when in full registry at each dead center surface portion at the same point of said rotary movement, controlling compression pressure change in one transition portion and decompression pressure change in the other transition portion and open at each main port portion, one having high pressure and the other having low pressure; each timing valve means providing a progressively variable length passage between a variable transition point in each transition surface and the one end of the adjacent main port portion dividing the transition surface portion into a pressure change surface portion extending to the other end of the adjacent dead center portion and a port extension portion extending to the one end of the adjacent main port portion operative to close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change anD to open each cylinder port to said adjacent main port portion during cylinder port movement over said port extension portion and movable to a minimum position to progressively lengthen said passage and move said transition point toward said adjacent dead center portion for minimum pressure change movement and pressure change and to maximum position to progressively shorten said passage and more said transition point toward said adjacent main port portion for maximum pressure change movement and pressure change to variably control the amount of pressure change and control means connected to the main port at high pressure and each of said timing valve means to independently control each of said timing valve means to control the amount of pressure change to provide a larger compression than decompression pressure change movement as a function of main port high pressure to make the cylinder pressure substantially equal to main port pressure at the transition point for equalized pressure interconnection.
 4. In a hydrostatic pump or motor unit; hydrostatic displacement means including stator means having cam means and rotor means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead center position at top and bottom dead center positions of said rotor means during rotation of said rotor means; displacement control means operatively connected to said cam means to vary the displacement of said hydrostatic displacement means from neutral; said stator means having a planar annular valve face having structurally arranged in annular sequence in one direction a first dead center closure surface portion, a first transition surface portion having a first timing valve means, a first main port portion, a second dead center closure portion, a second transition surface portion having a second timing valve means and a second main port portion; said rotor means having an annular port face in surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylinder port being connected to a cylinder and traversing said valve face portions during rotation in said one direction in the same sequence and during rotation in the opposite direction in the opposite sequence and being closed at each dead center surface portion, controlling compression pressure change in one transition portion and decompression pressure change in the other transition portion and open at each main port portion one having high pressure and the other having low pressure; each timing valve means providing a variable length passage between a variable transition point in each transition surface and the adjacent main port portion dividing the transition surface portion into a pressure change surface portion extending to the adjacent dead center portion and a port extension portion extending to the adjacent main port portion operative to continuously close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change and to continuously open each cylinder port to said adjacent main port portion during cylinder port movement over said port extension portion and movable to a minimum position to lengthen said passage and move said transition point toward said adjacent dead center portion for minimum pressure change movement and pressure change and to maximum position to shorten said passage and move said transition point toward said adjacent main port portion for maximum pressure change movement and pressure change to variably control the amount of pressure change movement and control means connected to the main port at high pressure, said timing valve means, said displacement control means and said rotor means to independently control said timing valve means to control the amount of pressure change to provide a larger compression than decompression pressure change movement as a function of main port high pressure, reduce both as a function of displacement variatioN from neutral, to reduce compression movement as a function of rotor speed and increase decompression movement as a function of rotor speed to make the cylinder pressure substantially equal to main port pressure at the transition point for equalized pressure interconnection.
 5. In a hydrostatic pump or motor unit; hydrostatic displacement means including stator means having cam means and rotor means having pistons and cylinders mounted for relative reciprocation by said cam means between a top and bottom dead center position at top and bottom dead center positions of said rotor means during rotation of said rotor means; displacement control means operatively connected to said cam means to provide neutral zero displacement and forward and reverse ranges of displacement; said stator means having a planar annular valve face having structurally arranged in annular adjacent sequence in one direction a first dead center closure surface portion, a first transition surface portion having a first timing valve means, a first main port portion, a second dead center closure portion, a second transition surface portion having a second timing valve means and a second main port portion; said rotor means having an annular port face in surface sealing rotary contact with said valve face having an annular series of cylinder ports; each cylinder port being connected to a cylinder and traversing said valve face portions during rotation in said one direction in the same sequence and being closed at each dead center surface portion, controlling compression or decompresssion pressure change in forward or reverse displacement in each transition portion and open at each main port portion each having low or high pressure in forward or reverse; each timing valve means providing a variable length passage between a variable transition point in each transition surface and the adjacent main port portion dividing the transition surface portion into a pressure change surface portion extending to the adjacent dead center portion and a port extension portion extending to the adjacent main port portion operative to close each cylinder port during cylinder port movement over said pressure change portion providing a stroke induced cylinder pressure change and to open each cylinder port to said adjacent main port portion during cylinder port movement over said port extension portion and movable to a minimum position to lengthen said passage and move said transition point toward said adjacent dead center portion for minimum pressure change movement and pressure change and to maximum position to progressively shorten said passage and move said transition point toward said adjacent main port portion for maximum pressure change movement and pressure change to variably control the amount of pressure change movement and timing control means connected to the ports and said timing valve means to independently control each of said timing valve means as a function of the higher pressure in either main port and as a function of the pressure in the adjacent main port to control the amount of pressure change to provide a larger compression than decompression pressure change movement as a function of main port high pressure to make the cylinder pressure substantially equal to main port pressure at the transition point for equalized pressure interconnection.
 6. The invention defined in claim 5 and said timing control means connected to said displacement control means to reduce compression and decompression pressure change movement as a function of the amount of forward or reverse displacement.
 7. The invention defined in claim 6 and said timing control means connected to said rotor and operative to reduce pressure change movement during compression and increase pressure change movement during decompression as a function of increasing rotor speed.
 8. In a hydrostatic pump or motor unit; housing means having a unit axis and a circular valve surface angularly fixed about said axis; said valve surface having sequentialLy adjacent in one direction of rotation a first main port, a first dead center closure portion, a first pressure change portion a second main port, a second dead center closure portion, a second pressure change portion and a second main port; hydrostatic displacement means including a rotor with a port surface rotatably mounted in continuous surface to surface sealing engagement on said valve surface, and a plurality of piston and cylinder means each having a cylinder port in said port surface and operative during rotation of said rotor to reciprocally stroke between a minimum volume top dead center position sequentially locating and closing said cylinder ports on one dead center closure portion always at the same top dead center point of rotation and a maximum volume bottom dead center position sequentially locating and closing said cylinder ports on the other dead center position always at the same bottom dead center point of rotation; said cylinder ports on moving across said pressure change portions providing when the cylinder port is closed a stroke induced cylinder pressure change; a timing valve means in each pressure change portion operative in a minimum pressure change position to provide minimum pressure change movement with the cylinder port remaining closed by a continuous surface in the part of said pressure change portion adjacent said dead center portion to cause a minimum stroke induced cylinder pressure change and during movement over the other part of said pressure change portion next to the adjacent main port to continuously connect said cylinder port to said adjacent main port and to provide for valve movement to a maximum pressure change position progressively increasing pressure change movement with the cylinder port remaining closed by an essentially continuous surface in the remaining part of said pressure change portion until initially connected to said adjacent main port to progressively increase stroke induced cylinder pressure change to a maximum directly as a function of pressure change movement and control means connected to said timing valve means to control said timing valve means to equalize cylinder pressure with main port pressure at the moment of interconnection.
 9. The invention defined in claim 8 and one of said main ports having high pressure and the other of said main ports having low pressure; means connected to the main port having high pressure to supply a high main port pressure signal varying with high pressure to said control means and said control means being operative in response to said high pressure signal to provide increased pressure change movement in both timing valves and a greater increase of pressure change movement by the timing valve controlling compression than the timing valve controlling decompression as a function of increasing high main port pressure.
 10. The invention defined in claim 8 and each timing valve means during movement from a minimum pressure change position toward a maximum pressure change position providing on initially connecting said cylinder port to said adjacent main port a gradually increasing flow capacity passage with increasing pressure change movement.
 11. The invention defined in claim 8 and each timing valve means being at the leading edge of the adjacent main port relative to cylinder port rotation in a pump.
 12. The invention defined in claim 8 and each timing valve means being at the trailing edge of the adjacent main port relative to cylinder port rotation in a motor.
 13. In a hydrostatic pump or motor unit; housing means having a unit axis and a transverse angularly fixed planar annular valve face having two partial semi-annular main ports annularly arranged with two pairs of facing ends and an intermediate planar face portion between each pair of facing ends; each intermediate face portion having, extending sequentially in the same rotary direction from its one end adjacent one facing end, a dead center closure portion and then a pressure change portion extending to its other end adjacent the other facing end; drive means; a cylinder block operably connected to said drive means and rotatably mounted in said housing for rotation about the unit axis and having a transverse planar annular port face in rotary sealing engagement with said valve face and an annular series of axial cylinders each having a cylinder port in said cylinder block port face sequentially connected to one of said main ports, controlled by one of said intermediate face portions, connected to the other of said main ports and controlled by the other of said intermediate portions during rotation of said cylinder block; a piston in each cylinder; piston reciprocating stroke means mounted on said housing and operatively connected to said pistons to sequentially reciprocate each piston between minimum volume top dead center position and maximum volume bottom dead center positions and locate and close each cylinder port in each dead center position on a closure portion always at the same rotary position and move each cylinder port across each pressure change portion providing stroke induced cylinder pressure change during rotation of said cylinder block; a timing valve means in each pressure change portion operative in a minimum pressure change position to provide a long fluid passage from the planar surface of the pressure change portion extending from a beginning point near said dead center closure portion to said other end of said closure portion and connected to said other end of the adjacent main port to provide minimum pressure change movement with the cylinder port remaining closed by said planar closure portion and planar part of said pressure change portion for minimum stroke induced cylinder pressure change and then to connect said cylinder by said fluid passage to said other end of the adjacent main port and operative on movement to a maximum pressure change position to progressively shorten said fluid passage by moving said beginning point toward said other end of the adjacent main port and extend said pressure change portion essentially as a planar surface to increase to maximum the pressure change movement with the cylinder port closed and stroke induced cylinder pressure change and then connect said cylinder by the shortened fluid passage to said other end of the adjacent main port and control means operatively connected to each of said timing valve means to control the amount of pressure change movement to control cylinder pressure to substantially match the pressure in the main port on interconnection of each cylinder port to a main port.
 14. The invention defined in claim 13 and said unit being a pump; said cylinder block rotating in one direction and each of said pressure change portions and said timing valve means being between a dead center position closure portion and the adjacent leading end of a main port.
 15. The invention defined in claim 13 and said unit being a motor; said cylinder block rotating in one direction and each of said pressure change portions and said timing valve means being between a dead center position and the adjacent trailing end of a main port.
 16. The invention defined in claim 13 and each of said timing valve means being a rotary cylindrical valve rotatably mounted in said housing means for rotation from said minimum pressure change position to said maximum pressure change position and having a valve face flush with said valve face and a peripheral recess in said valve face providing said long passage in said minimum pressure change position and said short passage in said maximum pressure change position.
 17. The invention defined in claim 13 and each timing valve means during movement from minimum pressure change position toward maximum pressure change position providing a gradually increasing flow capacity passage means for interchange of fluid between said cylinder port and said other adjacent end of the main port spaced from said dead center position.
 18. The invention defined in claim 13 and each of said tiMing valve means being a rotary cylindrical timing valve rotatably mounted in said housing means for rotation from said minimum pressure change position to said maximum pressure change position and having a face flush with said valve face and a recess in said face shaped to provide during said rotation said gradually increasing flow capacity passage.
 19. The invention defined in claim 13 and said timing valve means including a bore in said housing means located adjacent said valve face and extending from said other adjacent end of a main port past said dead center closure portion to the outer perimeter of said valve face, a series of openings extending from said valve face between said dead center closure portion to said adjacent end of said main port in the path of said pressure change movement of said cylinder port cumulatively connecting said cylinder port during rotation to said other adjacent end of a main port and a valve member movable in said bore from a minimum pressure change position opening a large number of openings to said bore for flow only to said other adjacent end of a main port to a maximum pressure change position cumulatively closing openings to shorten said passage through said openings to said other adjacent end of a main port.
 20. The invention defined in claim 19 and said series of openings consisting of two rows of longitudinally overlapping openings so the fluid flow capacity of the fluid passage between the cylinder port and the other adjacent end of a main port varies smoothly without undulation relative to rotation and the openings are small, shallow and sealed by the valve member to provide an essentially planar surface in the pressure change portion so pressure change varies smoothly without undulation relative to rotation.
 21. In a hydrostatic pump or motor unit; housing means having a unit axis and a circular valve surface angularly fixed about said axis; said valve surface having sequentially adjacent in one direction of rotation a first main port, a first dead center closure portion, a first pressure change portion a second main port, a second dead center closure portion, a second pressure change portion and a second main port; hydrostatic displacement means including a rotor with a port surface rotatably mounted in sealing engagement on said valve surface, and a plurality of piston and cylinder means each having a cylinder port in said port surface and operative during rotation of said rotor to reciprocally stroke between a minimum volume top dead center position sequentially locating and closing said cylinder port on one dead center closure portion always at the same rotary position during rotation of said rotor and a maximum volume bottom dead center position sequentially locating and closing said cylindrical ports on the other dead center position always at the same rotary position during rotation of said rotor; said cylinder ports on moving across said pressure change portions providing when the cylinder port is closed a stroke induced cylinder pressure compression of decompression change; a timing valve means in each pressure change portion operative in a minimum pressure change position to provide minimum pressure change movement with the cylinder port remaining closed in the part of said pressure change portion adjacent said dead center portion to cause a minimum stroke induced cylinder pressure change and during movement over the other part of said pressure change portion next to the adjacent main port to continuously connect said cylindrical port to said adjacent main port and to provide for valve movement to a maximum pressure change position progressively increasing pressure change movement with the cylinder port remaining closed in the remaining part of said pressure change portion to gradually increasing stroke induced cylinder pressure change to a maximum with maximum pressure change movement and control means operatively connected to said timing valve means to independently control each of said timing valve means tO provide a different amount of pressure change movement progressively varying as a function of the high main port pressure to equalize each cylinder pressure with each main port pressure at the moment of interconnection.
 22. The invention defined in claim 21 and governor means providing a governor signal varying with the speed of rotation of saud hydrostatic displacement means and said control means operatively connected to said governor means to provide a different amount of pressure change movement by the timing valve controlling compression than by the timing valve controlling decompression as a function of the speed of rotation.
 23. The invention defined in claim 21 and said hydrostatic displacement means including displacement varying means to vary the displacement providing a displacement signal varying as a function of displacement and said control means operatively connected to said displacement varying means operative to reduce pressure change movement by said timing valves as a function of displacement.
 24. The invention defined in claim 22 and said control means providing increased pressure change movement by the timing valve controlling compression and decreased pressure change movement by the timing valve controlling decompression with increasing speed.
 25. The invention defined in claim 24 and said hydrostatic displacement means including displacement varying means to vary the displacement providing a displacement signal varying as a function of displacement and said control means operatively connected to said displacement varying means operative to reduce the increase of pressure change movement due to high main port pressure.
 26. The invention defined in claim 25 and governor means providing a governor signal varying with the speed of rotation of said hydraulic displacement means and said control means correcting said increase of pressure change movement by said timing valves with said high pressure by increasing the pressure change movement of the timing valve providing decompression and decreasing the pressure change movement of the timing valve providing compression.
 27. In a hydrostatic pump or motor unit; housing means having a unit axis and a circular valve surface angularly fixed about said axis; said valve surface having sequentially adjacent in one direction of rotation a first main port, a first dead center closure portion, a first pressure change portion a second main port, a second dead center closure portion, a second pressure change portion and a second main port; hydrostatic displacement means including a rotor with a port surface rotatably mounted in sealing engagement on said valve surface, and a plurality of piston and cylinder means each having a cylinder port in said port surface and operative during rotation of said rotor to reciprocally stroke between a minimum volume top dead center position sequentially locating and closing said cylinder port on one dead center closure portion always at the same rotary position during rotation of said rotor and a maximum volume bottom dead center position sequentially locating and closing said cylinder ports on the other dead center position always at the same rotary position during rotation of said rotor; said cylinder ports on moving across one or the other of said pressure change portions providing when the cylinder port is closed a stroke induced cylinder pressure compression of decompression change; a timing valve means in each pressure change portion operative in a minimum pressure change position to provide minimum pressure change movement with the cylinder port remaining closed in the part of said pressure change portion adjacentg said dead center portion to cause a minimum stroke induced cylinder pressure change and during movement over the other part of said pressure change portion next to the adjacent main port to continuously connect said cylinder port to said adjacent main port and to provide for valve movement to a maximum pressure change position progressively increasiNg pressure change movement with the cylinder port remaining closed in the remaining part of said pressure change portion to progressively increase stroke induced cylinder pressure change to a maximum with pressure change movement and control means operatively connected to said timing valve means to provide more pressure change movement in the pressure compression change portion than the pressure change movement in the pressure decompression change portion to equalize each cylinder pressure with main port pressure at the moment of interconnection.
 28. The invention defined in claim 27 and one of said main ports having high pressure and the other main port having low pressure; means connected to said main port having high pressure to provide a high pressure signal varying with high pressure to said control means and said control means basically controlling said timing valve controlling compression to provide more pressure change movement than said timing valve providing decompression as a function of high pressure.
 29. In a hydrostatic pump or motor unit; housing means having a unit axis and a transverse angularly fixed planar annular valve face having two partial semi-annular main ports annularly arranged with two pairs of facing ends and an intermediate planar face portion between each pair of facing ends; each intermediate face portion having, extending sequentially in the same rotary direction from its one end adjacent one facing end, a dead center closure portion and then a pressure change portion extending to its other end adjacent the other facing end; drive means; a cylinder block operably connected to said drive means and rotatably mounted in said housing for rotation about the unit axis and having a transverse planar annular port face in rotary sealing engagement with said valve face and an annular series of axial cylinders each having a cylinder port in said cylinder block port face sequentially connected to one of said main ports, controlled by one of said intermediate face portions, connected to the other of said main ports and controlled by the other of said intermediate portions during rotation of said cylinder block; a piston in each cylinder; piston reciprocating stroke means mounted on said housing and operatively connected to said pistons to sequentially reciprocate each piston between minimum volume top dead center position and maximum volume bottom dead center positions and locate and close each cylinder port in each dead center position on a closure portion always at the same rotary position and move each cylinder port across each pressure change portion providing stroke induced cylinder pressure change during rotation of said cylinder block; a timing valve means in each pressure change portion operative in a minimum pressure change position to provide a fluid passage from the planar surface of the pressure change portion extending continuously from a beginning point near said dead center closure portion to its other end and said other facing end to provide minimum pressure change movement with the cylinder port closed for minimun stroke induced cylinder pressure change and then connect said cylinder by said fluid passage to said other facing end of a main port and operative on movement to a maximum pressure change position to progressively shorten said fluid passage by moving said beginning point toward said other end of said closure portion adjacent said other facing end to progressively increase to maximum the pressure change movement with the cylinder port closed and the stroke induced cylinder pressure change and then connect said cylinder by the shortened fluid passage to said other facing end and control means operatively connected to each of said timing valve means to independently and differentially control the amount of pressure change movement to control cylinder to substantially match the pressure in the main port on the interconnection of each cylinder port to a main port.
 30. The invention defined in claim 29 and one timing Valve means controlling compression and the other controlling decompression; one main port being a high pressure main port; high pressure signal means connected to said high pressure main port and said control means providing a high pressure signal proportional to said high pressure and said control means operative to provide greater pressure change movement controlled by the timing valve means controlling compression than the pressure change movement controlled by the timing valve means controlling decompression and both increasing with increasing high pressure.
 31. The invention defined in claim 29 one timing valve means controlling compression and the other timing valve controlling decompression; governor means operably connected to said cylinder block providing a governor signal varying with the speed of rotation connected to said control means and said control means operative to provide less pressure change movement controlled by the timing valve means controlling compression than the pressure change movement controlled by the timing valve means controlling decompression as a function of increasing speed.
 32. The invention defined in claim 29 and said piston reciprocating means including means to vary displacement and provide a displacement signal proportional to displacement to said control means and said control means reducing pressure change movement as a function of increasing displacement.
 33. The invention defined in claim 29 and one timing valve means controlling compression and the other controlling decompression; one main port being a high pressure main port; high pressure signal means connected to said high pressure main port providing a high pressure signal proportional to high pressure; said piston reciprocating means including displacement varying means to vary displacement and to provide a displacement signal proportional to displacement; and said control means operatively connected to said high pressure signal means and said displacement control means to provide greater pressure change movement controlled by the timing valve means controlling compression than the pressure change movement controlled by the timing valve means controlling decompression as a function of increasing high pressure, the pressure change movement of both timing valves increasing as a function of increasing high pressure and the increase reduced as a function of increasing displacement.
 34. The invention defined in claim 33 and governor means operably connected to said cylinder block providing a governor signal varying with the speed of rotation connected to said control means and said control means operative to reduce the increasing pressure change movement controlled by the timing valve means controlling compression and to increase the increasing pressure change movement controlled by the timing valve means controlling decompression as a function of increasing speed.
 35. In a hydrostatic pump or motor unit; housing means having a unit axis and a transverse angularly fixed planar annular valve face having two partial semi-annular main ports annularly arranged with two pairs of facing ends and an intermediate planar face portion between each pair of facing ends; each intermediate face portion having, extending sequentially in the same rotary direction from its one end adjacent one facing end, a dead center closure portion and then a pressure change portion extending to its other end adjacent the other facing end; drive means; a cylinder block operably connected to said drive means and rotatably mounted in said housing for rotation about the unit axis and having a transverse planar annular port face in rotary sealing engagement with said valve face and an annular series of axial cylinders each having a cylinder port in said cylinder block port face sequentially connected to one of said main ports, controlled by one of said intermediate face portions, connected to the other of said main ports and controlled by the other of said intermediate portions during rotation of said cylInder block; a piston in each cylinder; piston reciprocating stroke means mounted on said housing and operatively connected to said pistons to sequentially reciprocate each piston between minimum volume top dead center position and maximum volume bottom dead center positions and locate and close each cylinder port in each dead center position from said one facing end on a closure portion always at the same rotary position and move each cylinder port across each pressure change portion providing stroke induced cylinder pressure change during rotation of said cylinder block; a timing valve means in each pressure change portion each operative in all positions to maintain said closure portion in the same position relative to said one facing end so closure is always at the same rotary position, in a minimum pressure change position to provide a fluid passage from the planar surface of the pressure change portion extending continuously from a beginning point near said dead center closure portion to its other end and said other facing end to provide minimum pressure change movement with the cylinder port remaining closed for minimum stroke induced cylinder pressure change and then connect said cylinder by said fluid passage to said other facing end of a main port and operative on movement to a maximum pressure change position to shorten said fluid passage by moving said beginning point toward said other facing end to increase to maximum the pressure change movement with the cylinder port closed and the stroke induced cylinder pressure change and then connect said cylinder by any remaining shortened fluid passage to said other facing end and control means operatively connected to each of said timing valve means to control the amount of pressure change movement to control cylinder pressure to substantially match the pressure in the main port of the interconnection of each cylinder port to a main port.
 36. The invention defined in claim 35 and said control means connected to a main port and each of said timing valve means operative to provide differentially increased pressure change movement as a function of the higher main port pressure.
 37. The invention defined in claim 35 and displacement control means operatively connected to said piston reciprocating means to vary unit displacement and said control means connected to said timing valve means and said displacement control means to reduce pressure change movement as a function of increasing displacement.
 38. The invention defined in claim 35 and said control means including means responsive to rotary speed of said cylinder block to differentially control pressure change movement of each timing valve means as a function of speed.
 39. The invention defined in claim 35 and said control means connected to a main port and including means responsive to the rotary speed of said cylinder block and displacement control means to vary unit displacement operative to control each timing valve means to provide differentially increased pressure change movement as a function of high main port pressure with a reduction correction varying as a function of displacement and a differential function of speed. 